Increasing concerns over occupational health and safety and stringent regulations against solvent emissions have led to a desire to replace halogenated solvents in cleaning of machined metal parts. Standard replacements for halogenated molecules are formulated from propylene glycol ethers, but these solvents often lack broad solvency for both nonpolar and polar residues that are typically deposited in machining processes.
During machining processes of metal parts, oils and coolants are continuously applied to provide lubrication, cooling and removal of metal slag. Application of these machining fluids imparts improved tool life and enhances part surface finish by reducing metal-to-metal friction, thermal deformation and corrosion. Machining fluids are subdivided into four general categories: straight (or “cutting”) oils, soluble (emulsifiable) oils, full-synthetic coolants and semi-synthetic coolants. Straight oils are water immiscible and contain hydrotreated petroleum or mineral oils with a small fraction of polar lubricants (fats, esters, vegetable oils) and extreme pressure additives (typically organo-Cl, -S or -P compounds). Soluble oils are mixtures of 30-85% straight oils blended with surfactant-like emulsifiers. Stable emulsions are prepared by dispersing 3-10% of the soluble oil concentrate in water. Full-synthetic coolants contain no petroleum or mineral oils and are instead formulated from complex mixtures of alkaline inorganic and organic compounds. In order to improve part wetting and fluid performance, full synthetic fluids contain a wide variety of amines, surfactants, lubricants, biocides and corrosion inhibitors. The final working fluid is also prepared by diluting the full synthetic concentrate to 3-10% in water. Semi-synthetic fluids borrow the performance of both soluble oils and full synthetic coolants by blending 5-30% petroleum oil with full synthetic coolant and dispersing the mixture in 50-70% water. The selection of metalworking fluid is based on the desired lubricity and heat transfer performance at expected machining speeds and includes considerations such as metal compatibility and cost. In addition to these fluids, grinding pastes, polishing pastes and lubricating greases containing fatty acids, waxes and metal carbides and oxides are often used in machining processes. Highly acidic rosins, epoxy compounds and polar water-soluble fluxes are also extensively applied in soldering processes.
Subsequent to metalworking processes with any fluid, the machined metal parts can be contaminated with metal chips, oil-based residues, greases, lubricants, pastes and adventitious dust and dirt. Removal of these contaminants is often required prior to further processing (additional machining, painting, plating, heat treatments, assembly, etc.). Failure to clean the machined part can lead to film/coating adhesion difficulties, paint defects, blockage of tight tolerance spaces (threads, holes, etc.) and general poor final product quality. Historically, machined parts were cleaned by the so-called “cold-cleaning” methods—immersion, spraying, or wiping and rinsing in heated solvents. However, concerns with flammability, worker health, solvent emissions and poor cleaning performance and throughput led to the adoption of alternative solvents and methods. The second-generation liquid cleaners were predominately nonflammable halogenated solvents that were, at the time, considered safer replacements. Methylene chloride, trichloroethylene, perchloroethylene and n-propyl bromide were widely adopted as liquid cleaning solvents. Investigation of the long-term effects of these halogenated solvents led to increasing regulations due to toxicity, groundwater contamination and emission concerns. In order to eliminate the inherent environmental, health and safety (EHS) concerns with cold solvent cleaning, aqueous-based cleaning methods were also widely adopted. However, water-detergent based technologies were deficient in cleaning performance due to high water surface tension (˜2X of most organic solvents) and the tendency of many contaminants to hydrolyze, forming a difficult to remove soap and glycerol film on parts. In addition, these processes were found to be energy intensive and introduced the need for significant wastewater treatment and disposal.
Due to cleaning limitations with cold-cleaning processes, vapor degreasing was developed to provide enhanced cleaning performance. In a typical vapor degreasing process, the part to be cleaned is suspended in the vapor of a boiling solvent. The hot solvent vapor condenses on the initially cooler part and contaminants are removed by both physical entrainment and dissolution of machining fluid residues. The solvent-contaminant mixture is removed by gravity or mechanical rotation of the part. Once the temperature of the part reaches the vapor temperature, condensation ceases and the cleaning process is terminated. The vapor degreasing process enhances cleaning due to the generally higher cleaning temperatures and the reduced surface tension of the solvent in the vapor phase as compared to liquid. Lower surface tension facilitates solvent penetration into tight recesses of the part that would otherwise be inaccessible. In some instances, the cleaning process is augmented by immersion of the part or spray washing in hot solvent. The immersion cleaning step is often assisted by ultrasonic irradiation to impart a quasi-scrubbing action. Vapor degreasing technologies in use today include Open-Top Vapor Degreasers (OTVD), Closed-Loop Vapor Degreasers (CLVD), Vacuum Vapor Degreasers (VVD) and Airless Vacuum Vapor Degreasers (AVVD). OTVD, although still widely employed for parts cleaning using low boiling solvents, are open to the atmosphere and lead to significant worker exposure issues and large solvent emissions. As a result, solvent selection is critical to balance cleaning performance and EHS considerations along with the need to frequently replenish solvent losses. The other vapor degreasing technologies are inherently safer, closed cleaning systems, but concerns with personnel exposure and fugitive emissions are still present.
In addition to low surface tension, the solvent employed in vapor degreasing must have a vastly different boiling point than the contaminants that are removed to facilitate recovery and re-use of the cleaning solvent. Low water miscibility and resistance to unwanted reactions with water are highly desired to facilitate removal and solvent stability. Inherent water contamination occurs from atmospheric moisture and cleaning of water-based machining fluids. A heavy water layer, containing only a small fraction of solvent, is removed by physical decantation in gravity separators. The lighter solvent-rich layer, containing water to the miscibility limit and part contaminants, is returned to the solvent boiling sump for further use. In typical vapor degreasers, the solvent in the sump is continuously recovered and purified by vacuum distillation. The high boiling contaminants are then removed and the purified solvent is re-used multiple times without large changes in composition, boiling characteristics, or the need to replace or replenish solvent. As a result, the solvent must have a large relative volatility compared to typical machining fluids and exhibit thermal and chemical stability in the presence of these soils and over multiple cleaning and recovery cycles
Until the mid-1990s, five single component solvents were traditionally used is vapor degreasing processes: CFC-113, 1,1,1-trichloroethane (TCA), methylene chloride (MC), trichloroethylene (TCE) and perchloroethylene (PCE). Although possessing excellent solvency for both nonpolar and polar contaminants, CFC-113 and TCA were identified as potent ozone depleting chemicals and were subsequently banned. The chlorinated solvents MC, TCE and PCE are still employed but have inherent toxicity and worker exposure concerns, particularly in OTVD applications. As safer replacements for these chlorinated cleaners, several families of EHS compliant solvents were developed: the halogenated paraffinic hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), fluorinated olefins, halogenated oxygenates such as hydrofluoroethers (HFEs), fluorinated olefinic oxygenates and finally, fluorinated silanes. In recent years, many molecules in the HCFC series of solvents were also identified as ozone depleting and were largely phased out in January 2015 under the Montreal Protocol.
Although some halogenated single solvent systems have acceptable EHS profiles, the solvency of these molecules are largely paraffinic in nature, lacking solvency for soils with large hydrogen bonding and polar Hansen solubility parameters. In a response to increasing regulations and consumer perceptions against chlorinated solvents in particular and halogenated solvents in general, many non-halogenated cleaning solvents were developed that attempt to provide improved polar solvency. These solvents are typically single solvent systems, or simple binary or ternary zeotropic blends based on alcohols and/or glycol ethers, particularly propylene glycol monobutyl ether and propylene glycol monopropyl ether. The so-called “modified alcohol” solvents fulfill the need for nonflammable, low toxicity cleaners with zero ozone depleting potential and low global warming potential. Single solvent systems are simple to use and require no solvent formulation, but typically lack the flexibility of broad solvency for both nonpolar and polar contaminants. As a result, several binary and ternary zeotropic solvent blends were formulated to broaden the cleaning performance with a multi-component solvent. However, these zeotropic blends fractionate upon boiling, enriching the vapor in lower boiling components and modifying the cleaning power in the vapor phase. With solvent losses as vapor, the liquid composition of the solvent blend concentrates in higher boiling components with repeated use, thus modifying the effectiveness and boiling point of the solvent with time. Zeotropic solvent blends thus require regular solvent composition analysis and frequent solvent replacement or replenishment of the lost lower boiling components.
Simple binary and ternary zeotropic solvent blends are extensively employed in vapor degreasing, with the Dowclene™ series of solvents finding wide use. The Dowclene™ solvents are composed of blends of propylene glycol ethers (e.g., Dowclene™ 1601 is propylene glycol monobutyl ether (PnB) and dipropylene glycol dimethyl ether (DMM)). The addition of DMM serves to improve the nonpolar solvency character of PnB by effectively decreasing the polar and hydrogen bonding Hansen solubility parameters of the mixture. As a result, the zeotropic blend of PnB-DMM has a polar and hydrogen bonding solvency contribution due to PnB and a more nonpolar solvency contribution attributed to DMM. The net solvency of the blend, per volumetric blending rules, lies intermediate to both components. However, it is recognized that binary zeotropic degreasing solvents composed of the propylene glycol ethers lack adequate solvency for polar contaminants while depositing an opaque residue on cleaned parts. Similarly, it is known that other vapor degreasing solvents formulated for higher paraffinic solvation ability display poor solvency for polar soils and tend to deposit waxy residues. Binary zeotropic solvents are often re-formulated with addition of a third component to improve polar solvency. Generally, lower alcohols such as ethanol, n-propanol, isopropanol, n-butanol and t-butyl alcohol are employed for this purpose. Again, these solvent systems suffer from fractionation due to solvent emissions and thus require continuous monitoring of solvent composition.
Fractionation of cleaning solvent blends can be eliminated by utilizing a solvent mixture at its azeotropic composition. In this case, the solvent that is boiled has the same vapor composition as in the liquid phase and enrichment of the vapor phase in lower boiling components does not occur. As a result, the solvent blend behaves as a single component system with a constant composition and constant boiling point that cannot be separated by fractionation. Binary, two-component azeotropes are classified as minimum or maximum boiling where the boiling point of the azetrope boils at a temperature lower or higher than either pure component, respectively. Minimum boiling azeotropes can be further categorized as either homogeneous or heterogeneous, where the liquid forms a single phase or two separate phases. Furthermore, many binary azeotropes have compositions that are highly pressure dependent. The Clausius-Clapeyron equation relates the heat of vaporization of a compound to the slope of the vapor pressure curve as a function of temperature. As a result, the azeotropic composition of a blend of components with substantially different heats of vaporization will depend strongly on pressure. Large differences in heats of vaporization permit “breaking” an azeotrope at reduced pressures, analogous to pressure-swing azeotropic distillation. The aforementioned deficiencies with zeotropic solvent blends are introduced when employing pressure-dependent azeotropic solvents at pressures different from the intended conditions. Thus, a solvent formulated for vapor degreasing at one pressure will be far from the azeotrope pinch point at other pressures and effectively behave as a zeotropic solvent. This limits the available operating range of a pressure-dependent azeotropic solvent and necessitates formulation of multiple solvent compositions to tailor the blend to a desired operating pressure. The capability to manipulate operating pressures with maintenance of azeotropic behavior of the solvent is highly desired and lends flexibility to operation of vapor degreasing equipment.